Plackowska ordered the two children into the girl's bedroom, ordered them to get on their knees and pray, prosecutors said.

She told her son he was going to heaven that night and began stabbing him as the boy pleaded for his life, they said.

Plackowska then killed Olivia because she was a witness, they said.

Plackowska is charged with two counts of first-degree murder. She has no previous criminal history and has given authorities various explanations for what happened, including hearing demonic voices that led her to kill the children.

The children were discovered by police shortly after 10 p.m. Tuesday after the girl's mother Marta Dworakowski, a nurse, couldn't find her after working the evening shift at a dialysis lab, according to neighbors and officials.

The boy was on a mattress and the girl on the floor, officials said.

Plackowska -- also identified in public records as Plackowski -- told police she had taken the children with her to a church in Naperville earlier Tuesday, then returned to the Dworakowski home in the 800 block of Quin Court, according to a law enforcement source.

At some point Tuesday night, Plackowska argued on the phone with her husband because she wanted to return to her native Poland while he wanted to remain here, the source said.

Plackowska told police she attacked the children after hearing demonic voices, the source said. She told police she had to save the children's souls from the devil by killing them, according to the source.

When pressed, she told authorities she also was angry with her husband and distraught over the death of her father in Poland, the source said.

After the slayings, Plackowska drove back to the church but couldn't reach anyone, she allegedly told authorities. She also left several frantic voice messages on the church's answering machine before ultimately tossing her cellphone out the car window, officials said.

Plackowska, covered in blood, then drove to a home on Violet Circle in Naperville, where her adult son called 911, sources said. Police said they were alerted at 10:18 p.m. Tuesday and had to break into the Quin Court home. Investigators recovered a kitchen knife believed to have been used in the attack, a source said.

Michelle Jones, who lives in the same complex as Plackowska in the 200 block of East Bailey Road in Naperville, said she frequently saw Plackowska with Justin and his older sibling, and with Olivia.

Dworakowski also was a familiar face who regularly came to the building to scoop her daughter up at the end of the day, Jones said.

But by 11 p.m. Tuesday, Jones saw a panicked Dworakowski speaking with police in the building's lobby. "She was sitting out there trying to talk to the police, and then she started crying, and I'm like, 'Oh, I can't imagine what you're going through,'" Jones said. "It's 11 o'clock and you don't know where your kid is."

Another of Plackowska's neighbors, Angela Pombert, 32, said she also saw the distraught mother speaking with police. "I felt awful for them," Pombert said. "She was hysterical almost."

As the sun set, crime scene tape remained up on the garage and on the front of the home, while trick-or-treaters raced from house to house collecting candy.

Outside Plackowska's home, a young boy stood in front of the building, looking up at a balcony where his friend's Huffy Pro Thunder bike perched.

He called out, "Justin!" into the cold air before turning to run away.

As details of the killings on the quiet cul de sac slowly emerged, residents throughout the community reacted with shock. Some wondered how to carry on with regular Halloween activities, while others worried about how their children would react when they found out about the deaths.

"It's a tragedy," said Laura Maniaci of Naperville, who has two children at Scott Elementary School, where Justin was a student. "How do you explain this to your child? I don't know."

Children showed up in jellyfish and pumpkin costumes for the annual Halloween parade at the school, where students were offered counseling by crisis teams expected to remain as long as necessary to deal with the "tragic and upsetting event," said Naperville Unit District 203 Superintendent Dan Bridges.

Across town, counselors were also on hand for students at Brookdale Elementary, which confirmed the death of a kindergarten student on its website.

Tim Prerost, who has a daughter at Scott, said, "It's unthinkable, it's shocking, but we don't know what really happened."

Another parent who would identify herself only as Christine said, "It's horribly sad. It's a tightknit school and it's really difficult for everyone."

The Illinois Department of Children and Family Services also is investigating the Naperville slayings, and agency officials confirmed they did not have any prior contact with either family.